1. Field of the Invention
The invention relates generally to a rotary cutter earth boring bit, and more specifically to structure for retaining the rotary cutter on the journal bearing pin.
2. Description of the Prior Art
Throughout the development of the rotary cutter earth boring rock bit, many methods have been developed for retaining the cutter on the journal bearing pin. For the most part, the present commercially acceptable method of cutter retention requires the use of balls disposed in opposed axially aligned circumferential grooves formed within the bearing surface of the pin and the mating bearing cavity of the cutter. This requires a ball passage, formed within the pin, extending from exteriorly of the pin to the circumferential groove, for loading the balls within the groove subsequent to the cutter being assembled thereon. In addition to the balls occupying substantial axial space that might otherwise be used for greater journal bearing surface and capacity, during normal drilling operations the forces of the cutter force the cutter inward toward the center of the hole (in-thrust) and the balls are directly subjected to loading which can cause spalling or partial failure of the ball grooves which forms metal debris within the bearing cavity. This debris in turn can cause premature failure of the journal bearings. Also the balls and grooves can become worn to the extent that complete failure of the retention mechanism occurs whereby the cutter can be lost in the hole. In either instance, the results are rather catastrophic.
Further, the use of balls as retaining means presents problems associated with the ball passage. In this regard a hole must be formed through the bit arm to the ball groove in order to install the balls. This hole or passage is subsequently filled with a hand-fitted ball plug which must be welded in place. This hand-fitting requires a great amount of care, as does the welding process, adding to the manufacturing and assembly processes and allowing a greater possibility of faulty assembly which again leads to rather catastrophic results.
Although the prior art is replete with snap ring structure for retaining the cutter on the journal pin, heretofore none of this proposed structure has found commercial acceptability, generally for the reasons that it either adds rather complex structure to the assembly or permits the snap ring to be deformed or compressed during actual drilling operations, to a position where it no longer retains the cutter on the pin, thereby permitting the cutter to be released, neither of which are acceptable in this highly competitive field.
Reference is made to U.S. Pat. No. 3,746,405 which, for the most part, is directed to a lubrication and seal structure for a rotary cutter rock bit, but also discloses, with reference to FIG. 2, a cutter retention structure utilizing the balls disposed in opposed grooves as above discussed. However, with reference to FIGS. 5 and 6 thereof, a snap ring retainer is shown disposed within opposed grooves in the pin and cutter bearing surfaces. As therein seen, the ends of the snap ring, in its relaxed condition, are arcuately separated so that the ring can be deformed or compressed to a non-interfering position for assembly of the cutter over the pin. Once the cutter is assembled in the proper position and the opposed grooves aligned, the snap ring snaps outwardly to a cutter retention position. However, in such condition, the frictional forces during operation of the bit can cause the ring to again deform to a non-interfering position whereby the cutter will be released from the pin under any in-thrust loading.
U.S. Pat. No. 4,236,764 also discloses a snap ring received within opposed aligned grooves between the journal and the bearing surface of the cutter, with the pin groove having a geometry such that, as the cutter experiences in-thrust forces the retaining ring is, by virtue of the angled faces, forced to expand radially outwardly to an interfering condition that prevents the cutter from being released. However, again it is shown that the ends of the snap ring are separated by an arcuate space so that, once the ring is expanded to its relaxed condition within the opposed grooves to prevent release of the cutter, it is possible for the ring to be deformed to a non-interfering position by closure of this arcuate gap whereupon the cutter would be released. It has been found that if such action is at all possible, during drilling operations wherein various forces are encountered, there are circumstances where it will occasionally occur with loss of the cutter downhole.
Other prior art patents show snap rings or expandable rings received between opposed grooves on the journal and the cutter and which are prevented from collapse by other structure such as a threaded attachment subsequently assembled, or its equivalent, (see U.S. Pat. Nos. 2,654,577 and 2,579,819). However, such structure provides a generally weakened bearing assembly in addition to adding parts and labor to the overall structure. Other prior art devices include a lock nut such as shown in U.S. Pat. No. 3,971,600 and a welded thrust member which is welded onto the journal pin subsequent to the cone being assembled thereon such as shown in 4,176,724. In both these instances, the subsequent assembly techniques are difficult and time-consuming and the resultant structure susceptible to stress-induced premature failure.